1. Field of the Invention
The present invention relates generally to systems and methods for tuning and demodulating radio frequency (RF) signals, and more particularly, to a tuner providing complex digital signal channel selection and image rejection.
2. Description of Related Art
Analog cable television (also known as “CATV”) brings television programs to millions of viewers throughout the world. Analog cable television is transmitted using a radio frequency (RF) signal that comprises several channels or bands of signals. In order to effectively present a channel to a viewer, an electronic device, such as a tuner, is used to separate and process one channel for presentation.
Tuners may be fabricated on circuit boards and then installed in computer systems, thereby allowing the computer system to operate as a television set. Many tuners convert high frequency RF signals to one or more Intermediate Frequency (IF) signals which, at a later step, are converted to baseband signals. Such IF signals are at a lower frequency than the RF signals. Each translation stage normally uses mixing to produce both a desired signal and an image signal. If the image signal falls into the same IF frequency band as the desired signal, the image signal should be removed from the desired signal. This process of correcting the desired signal by removing the image signal is referred to as image rejection.
FIG. 1 is a prior art block diagram of a conventional tuner 100 for analog cable television. The conventional tuner 100 includes a mixer 105, a surface acoustic wave (SAW) filter 110, a local oscillator 115, an image rejection mixer 120, and a real analog signal channel select filter 125. In operation, the mixer 105 combines an RF signal (comprised of image and signal components) with a first oscillator signal 130 received from the local oscillator 115 to generate a high intermediate frequency (IF) signal 135 of approximately 1.0 GHz. The high IF signal 135 is sent to the SAW filter 110 for image rejection processing. Typically, the SAW filter 110 suppresses the image component of the high IF signal 135 by approximately 30 dB. The SAW filter 110 is typically located off-chip (i.e., not integrated on-chip with other components of the conventional tuner 100), since the SAW filter 110 is configured as a high-Q filter for processing the high IF signal 135. The filtered high IF signal 140 is split and sent to mixers 145A and 145B of the image rejection mixer 120. The image rejection mixer 120 also comprises ninety-degree phase delay modules 150A and 150B and a summer 155. In operation, the image rejection mixer 120 down-converts the filtered high IF signal 140 and provides an additional 30 dB of image rejection to generate a low IF signal 160. Ideally, the conventional tuner 100 receives the RF signal with image and signal components, filters the image component off-chip, generates a low IF signal 160 comprised of a signal component and a small or nonexistent image component, and transmits the low IF signal 160 to the real analog signal channel select filter 125.
FIG. 2A is a prior art plot of spectral amplitude S(f) of the low IF signal 160 illustrated in FIG. 1. The dotted line represents an image component 205 of the RF signal that has been suppressed or removed from the RF signal spectrum by the SAW filter 110 (FIG. 1) and the image rejection mixer 120 (FIG. 1). In other words, the conventional tuner 100 generates the low IF signal 160 comprised of a signal component 210 only. Since the image component 205 is removed from the RF signal, the real analog signal channel select filter 125 (FIG. 1) is typically configured as a real filter to select an appropriate signal channel (and a corresponding non-existent image channel) from the low IF signal 160. As is known in the art, pairs of poles in the complex frequency plane define a real filter, where a first pole of a pair of poles is a complex conjugate of a second pole of the pair of poles.
FIG. 2B illustrates a prior art frequency response A(f) (i.e., magnitude of a transfer function) of the real analog signal channel select filter 125 of FIG. 1. As is known to one skilled in the art, the real analog signal channel select filter 125 passes (i.e., selects) pairs of positive and negative frequency bandwidths, such as a negative frequency bandwidth 215 and a positive frequency bandwidth 220. However, since the image component 205 (FIG. 2A) of the low IF signal 160 (FIG. 1) received by the real analog signal channel select filter 125 is suppressed, the filter 125 only passes a portion of the signal component 210 (FIG. 2A) that lies within both the positive and negative frequency bandwidth 220. Real signal channel select filters of the prior art are typically implemented as external (i.e., off-chip) SAW filters.
Conventional tuners typically provide approximately 60 dB image rejection and signal channel selection by using integrated tuner components and off-chip fixed filters, such as external SAW filters. Such off-chip filters require additional pins and interface components, thus increasing power consumption, packaging costs, and placing electrical and physical design constraints on other tuner components. Some conventional tuners have been configured with on-chip analog filters to perform image rejection; however, such tuners require costly and complicated circuitry to provide desired signal accuracy. There exists a need for a fully integrated analog cable television tuner that provides image rejection and digital signal channel selection.